In general, hollow metal members are employed as components of industrial equipment, transport equipment, etc. and, for example, they are widely employed as frame members such as body frames or door frames in automobiles.
In recent years, accompanying demands for environmental protection measures, recycling, savings in resources, weight reduction, etc., the hollow members have employed a lightweight material such as an aluminum material, and there is also a desire for the development of a tubular member having its wall thickness and cross-sectional shape freely controllable in the longitudinal direction and having surplus material cut out so as to give an optimum wall thickness distribution, and a hollow member having an optimum cross-sectional shape in the longitudinal direction.
For example, Japanese Patent Application Laid-open No. 10-230318 discloses a process for producing a hollow member having cross-sectional shape variation in the longitudinal direction by bulge forming a hollow material that has been extruded using a die and a mandrel in combination.
Furthermore, Japanese Patent Application Laid-open No. 5-76950 and Japanese Patent No. 2874467 disclose processes for producing a hollow member in which, after a predetermined part of a tubular material having a uniform wall thickness is heated, the tubular material is compressed in the longitudinal direction so as to increase the thickness of the heated portion, thus giving a hollow member having a cross-sectional shape that varies in the longitudinal direction.
However, the process disclosed in Japanese Patent Application Laid-open No. 10-230318 is not only incapable of optimally controlling the wall thickness distribution in the longitudinal direction, but also requires special extrusion equipment in order to make the cross section of the hollow member variable, thereby giving rise to the problems of the equipment being large scale, the equipment cost being high, the productivity being poor, and the process being difficult to put into practice.
Moreover, in the processes disclosed in Japanese Patent Application Laid-open No. 5-76950 and Japanese Patent No. 2874467, since the tubular material is compressed in its longitudinal direction, there is the problem that a high precision product cannot be obtained because, for example,    (1) there is a possibility that the tubular material might buckle, collapse, etc.    (2) it is difficult to make the circumference of the tubular material uniform along its whole length.